Research is concerned with the behavior of afferents from muscle spindles in the jaw closing muscles of monkeys during controlled, voluntary movements of the mandible. Monkeys are trained to hold their mandible within a specified target zone of incisal opening. The activity of cells in the mesencephalic nucleus of the fitfth nerve is recorded, using extracellular micro-electrodes, during this behavior. Preliminary data indicates that afferent fibers, provisionally identified as secondary endings of the basis of their behavior, exhibit a rate of discharge that is substantially and linearly related to the amount of jaw opening, and which is independent of the amount of force necessary to maintain the jaw position. Another class of jaw muscle spindle afferent, provisionally identified as primary endings, discharge at a rate that is unrelated to static jaw position, but that increases dramatically for jaw-opening movements. The typical rate of discharge of primary endings increases substantially at all static postions when the force necessary to maintain the position is increased in the range of 10 to 200 grams.